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Silt Density Index - SDI

Membrane Fouling Control

Membrane fouling is the main cause of permeate flux decline and loss of product quality in reverse osmosis (RO) Systems, so fouling control dominates RO system design and operation. Sources of fouling can be divided into four principal categories: scale, silt (particular), bacteria (bio fouling, growth of bacteria) and organic fouling (oil, grease).

Fouling control involves pre-treatment of the feed water to minimize fouling as well as regular cleaning to handle fouling that still occurs. Fouling by particulates (silt), bacteria and organics generally affects the first modules in the plant the most. Scaling is worse with more concentrated feed solutions, therefore the last modules in the plant are most affected, because they are exposed to the most concentrated feed water.

Silt density Index

Silt is composed by suspended particulates of all types that accumulate on the membrane surface. Sources of silt are organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particular matter. Silt Density Index testing is a widely accepted method for estimating the rate at which colloidal and particle fouling will occur in water purification systems, especially using reverse osmosis (RO) or Nanofiltration membranes.

SDI is a measurement of the fouling potential of suspended solids. It’s not measuring the quantity of particular matter, since the size, shape vary. Turbidity is a measurement of the amount of suspended solids. They are not the same and there is no direct correlation between them. In practical terms however, the membranes show very little fouling when the feed water has a turbidity of < 1 NTU. Correspondingly the membranes show very low fouling at a feed SDI of less than 5.

The SDI test is used to predict and then prevent the particulate fouling on the membrane surface. Other names for it are the Kolloid-Index (KI) or the Fouling-Index (FI). The test is defined in ASTM Standard D4189, the American Standard for Testing Material.

It measures the time required to filter a fixed volume of water through a standard 0.45µm pore size microfiltration membrane with a constant given pressure of 30 psi (2,07 bar). The difference between the initial time and the time of a second measurement after normally 15 minutes (after silt-built up) represents the SDI value.

Figure 1: the Silt Density Index (SDI)



¼ “ Ball valve


Pressure regulator (e.g. needle valve)


Pressure gauge 0-5 bar


Filter holder Æ 47mm


0,45 µm filter Æ 47mm


500 ml graduated cylinder







The feed water has to be supplied with a pressure of 2 bars, which will be regulated with the pressure regulator and reading off the value on the pressure gauge.

With the ball valve you turn the flow on and off.

The filter holder contains the 0,45 µm filter with a diameter of 47mm, which is more likely to clog from colloidal matter than from hard particles such as sand or scale.

With this equipment you can measure the amount of time required for 500 ml of feed water to flow through the filter.

The water continues flowing with a pressure of 2 bars through the filter.

After 5, 10 and 15 minutes you can measure again the time required for 500 ml to pass the filter. The 15-minute index will generally be the lowest of the three, and should be used for filter sizing purpose.

The 15-minute SDI (SDI 15) is defined by ASTM as the interval required for accurate and standardized testing. The 5- and 10-minute SDI values are only estimates of the 15-minute value, although the most people advice to measure all three measurements.


ð There is no correction factor or correlation for running the SDI test at pressures other than 30 psi. However, the SDI value at 20 psi, should still be useful. %P30 is a term used for plugging factor at 30 psi. It equals SDI multiplied by the duration of the test [standard 15 minutes]. [4]

ð The SDI may vary as a function of water temperature and values obtained at different temperatures may not necessarily be comparable! The water temperature must remain constant (± 1ºC) throughout the test. This is necessary as flow rate changes by about 3% per ºC. [4]

ð The SDI will vary with the membrane filter manufacturer. This, SDI values obtained with filters from different membrane manufacturers, cannot be comparable.[4]

How to Calculate Silt Density Index and Plugging Factor?

After completion the test, calculate the SDI by using the equation below.

ti = Initial time in seconds required to collect the 500 ml sample

tf = Final time in seconds required to collect the second sample after test time

tt = total elapsed test time (either 5, 10 or 15 min)

The maximum 5-minute SDI equals 100% divided by 5 minutes, which gives you 20 SDI units (percent decay per minute). The same with 10 and 15 minutes.


SDI Range

5 Minute

0 - 20

10 Minute

0 - 10

15 Minute

0 - 6.67

The SDI values give the following indications for reverse osmosis:

SDI < 1

Several years without colloidal fouling

SDI < 3

Several months between cleaning

SDI 3 – 5

Particular fouling likely a problem, frequent cleaning

SDI > 5

Unacceptable, additional pre-treatment is needed

Also varies with the module design:

Spiral-wound modules as used in the Nanofiltration test generally require an SDI<5, whereas hollow fine fibre modules are more susceptible to fouling and require an SDI<3.

The target SDI after filtration is normally an SDI of 3 – 5 or less. Surface or seawater may have an SDI up to 200, requiring flocculation, coagulation, and deep-bed multimedia filtration before RO treatment.

In addition the decay in flow rate is also converted from an SDI value to a plugging factor (PF) value, which express the plugging with a percentage, where 100% means the filter is completely plugged.

Example: SDI15 = 1.4 SDI units, then PF = 1.4 / 6.7 * 100, or 21% plugged.

Also you can calculate:


Report the following information:

- The SDI, with a subscript indicating the total elapsed flow time (T) in minutes.

- The water temperature before and after the test

- The manufacturer of the filter used for the test, as well as the manufacturer's identification for the membrane filter.

see also on Lenntech web-page




Related topics

Sources: "Membrane Technology and Applications”, Richard W. Baker, Wiley, 2004 Start: Page 216

Prospect: polysep industrial consultants





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